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    <title>ShellCheck: SC2199 – Arrays implicitly concatenate in `[[ ]]`. Use a loop (or explicit `*` instead of `@`).</title>
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    <h1>SC2199 – ShellCheck Wiki</h1>
    <a href="https://github.com/koalaman/shellcheck/wiki/SC2199">See this page on GitHub</a>
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    <h2
id="arrays-implicitly-concatenate-in---use-a-loop-or-explicit--instead-of-">Arrays
implicitly concatenate in <code>[[ ]]</code>. Use a loop (or explicit
<code>*</code> instead of <code>@</code>).</h2>
<h3 id="problematic-code">Problematic code:</h3>
<div class="sourceCode" id="cb1"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb1-1"><a href="SC2199.html#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="va">ext</span><span class="op">=</span>png</span>
<span id="cb1-2"><a href="SC2199.html#cb1-2" aria-hidden="true" tabindex="-1"></a><span class="va">allowedExt</span><span class="op">=</span><span class="va">(</span>jpg bmp png<span class="va">)</span></span>
<span id="cb1-3"><a href="SC2199.html#cb1-3" aria-hidden="true" tabindex="-1"></a><span class="kw">[[</span> <span class="st">&quot;</span><span class="va">$ext</span><span class="st">&quot;</span> <span class="ot">=</span> <span class="st">&quot;</span><span class="va">${allowedExt</span><span class="op">[@]</span><span class="va">}</span><span class="st">&quot;</span> <span class="kw">]]</span> <span class="kw">&amp;&amp;</span> <span class="bu">echo</span> <span class="st">&quot;Extension is valid&quot;</span></span></code></pre></div>
<h3 id="correct-code">Correct code:</h3>
<div class="sourceCode" id="cb2"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb2-1"><a href="SC2199.html#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="va">ext</span><span class="op">=</span>png</span>
<span id="cb2-2"><a href="SC2199.html#cb2-2" aria-hidden="true" tabindex="-1"></a><span class="va">allowedExt</span><span class="op">=</span><span class="va">(</span>jpg bmp png<span class="va">)</span></span>
<span id="cb2-3"><a href="SC2199.html#cb2-3" aria-hidden="true" tabindex="-1"></a><span class="cf">for</span> value <span class="kw">in</span> <span class="st">&quot;</span><span class="va">${allowedExt</span><span class="op">[@]</span><span class="va">}</span><span class="st">&quot;</span></span>
<span id="cb2-4"><a href="SC2199.html#cb2-4" aria-hidden="true" tabindex="-1"></a><span class="cf">do</span></span>
<span id="cb2-5"><a href="SC2199.html#cb2-5" aria-hidden="true" tabindex="-1"></a>  <span class="kw">[[</span> <span class="st">&quot;</span><span class="va">$ext</span><span class="st">&quot;</span> <span class="ot">=</span> <span class="st">&quot;</span><span class="va">$value</span><span class="st">&quot;</span> <span class="kw">]]</span> <span class="kw">&amp;&amp;</span> <span class="bu">echo</span> <span class="st">&quot;Extension is valid&quot;</span></span>
<span id="cb2-6"><a href="SC2199.html#cb2-6" aria-hidden="true" tabindex="-1"></a><span class="cf">done</span></span></code></pre></div>
<h3 id="rationale">Rationale:</h3>
<p>Array expansions in <code>[[ .. ]]</code> will implicitly concatenate
into a single string, much like in assignments. The problematic code is
equivalent to <code>[ "$ext" = "jpg bmp png" ]</code>.</p>
<p>Instead, use a <code>for</code> loop to iterate over values, and
apply your condition to each.</p>
<p>Alternatively, if you do want to concatenate all the values in the
array into a single string for your test, use <code>"$*"</code> or
<code>"${array[*]}"</code> to make this explicit.</p>
<h3 id="exceptions">Exceptions:</h3>
<p>None.</p>
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